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It has been 10 years since University of Pennsylvania psychologist Martin Seligman kick-started the field of Positive Psychology  the study of what makes people happy, what makes life fulfilling and the role of positive emotions in the human psyche. (Traditionally, of course, psychology has focused on the reverse: sadness, anxiety, anger, grief.)

The decade mark was observed last month in Philadelphia by the First World Congress on Positive Psychology, where it was clear that the field is flourishing, to use a favorite word of positive psychologists. Planners had hoped for 800 attendees and got twice that number, with psychologists, educators and students from more than 50 countries descending upon the City of Brotherly Love. There were signs that findings by positive psychology researchers had begun to influence economics, education and even government policy in some countries. But it was also clear that some of the heady findings from the infancy of the field had been oversimplified, and that later research has become more nuanced. (See 20 ways to get and stay healthy.)

Early work by positive psychologists had established intriguing correlations between happiness or optimism and factors like wealth, marriage, health and longevity, but there was little in the way of rigorous science to explain these associations. Now that's beginning to change. At Carnegie Mellon, for instance, psychologist Sheldon Cohen has been exploring exactly how positive emotions affect the body. (This is the flip side of previous work by Cohen and others linking stress, Type-A behavior and negative emotions to lowered immunity, heart disease and shorter lifespan.) Cohen's research shows that people with a "positive emotional style" have better immunity to cold and influenza viruses when exposed in the lab. His most recent work, presented at the conference, suggests that this is mainly due to the release of optimal levels of cytokines, proteins that regulate the immune response.

Cohen and his colleagues have also been studying how social relationships and positive emotions can impact lifespan. Their work builds on a famous 2001 University of Kentucky study of aging nuns, which found that the more positive emotions the nuns had expressed in brief autobiographies written 60 years earlier at age 22, the longer they lived. In an interesting twist on that study, Cohen and colleague Sarah Pressman similarly analyzed a collection of autobiographies  this time, written by 96 leading psychologists at an average age of 65. Once again, there was a correlation between longevity and positive emotions, but in the newer study the relationship held only for "active" expressions of emotion, such as "excited," "thrilled" and "delighted" as opposed to passive emotions like "pleased" and "calm." Falling in line with other recent social research in the elderly, the analysis found that language indicating strong social relationships was powerfully associated with longer life. (See how to prevent illness at any age.)

Studies like Cohen's help ground a young and sometimes fluffy field in hard science, says conference chairman Chris Peterson of the University of Michigan. "It's very, very careful, creative research," he says, while conceding, "There's a temptation to bullshit in positive psychology."

That temptation was addressed head-on in a keynote speech by Ed Diener, president of the International Positive Psychology Association. Diener pointed out that some of the most popular findings on happiness have not held up to further study, that researchers have begun to the record straight.

One such study had found that people who win lotteries are no more satisfied with their lives after winning than before. Another purported to show that people who became paraplegics were able to return to their previous level of happiness within a few years after their disabling accident. "We want these things to be true, but they are not quite entirely true," said Diener."

"There are unhappy lottery winners, but generally if a poor person wins the lottery, they are a little happier with their life," noted Diener, who is known as "Dr. Happiness" for his foundational work in the field and who holds the aptly named Smiley chair in psychology at the University of Illinois. As for paraplegics, "there is a big drop for those who became 100% disabled, meaning they can no longer do any work." In general, Diener noted, people do adapt to a major life change but not completely. "We have to be careful when we cite these studies," warned the genial researcher, who wore a smiley face T-shirt under his sport coat.

Such findings support a widely held theory by happiness researchers that a person's level of satisfaction is determined largely by character and attitudes  less by external factors, like money or disability  and that we tend to return to our personal set point. But another branch of research  the one that leads to bestselling books and, at the conference, sessions that were packed to the point of fire-code violation  suggests that set point can be modified, and that people can learn to be happier.

Seligman has pioneered a number of well-publicized happiness-boosting exercises, for example: keeping a gratitude journal, jotting down three good things or "blessings" that occur each day, making a practice of doing "acts of kindness" for others, writing a letter of gratitude to a mentor. But more recent research, particularly by Sonja Lyubomirsky at University of California, Riverside, indicates that some of these exercises can lose their power with too much repetition: "They become stale and stagnant," says Lyubomirsky.

In one study, Lyubomirsky found that people who did the same acts of kindness day after day for 10 weeks actually got less happy. But those who systematically varied their good deeds, got a boost. And, logically enough, effort counted. Letters of gratitude that reflected serious effort brought more satisfaction than those that seemed written routinely or out of duty. On the other hand, Lyubomirsky has also found that people who are clinically depressed "got less happy writing gratitude letters," perhaps because the effort was too much.

Just as there is no universal formula for treating the psychological conditions that plague us  depression, anxiety, stress  there's no one-size-fits-all trick to boosting happiness. In her recent book, The How of Happiness, Lyubomirsky aims to help joy-seekers find activities that are their best personal match. But for those who are better suited to technology than book-reading, she's just unveiled another tool, which is perhaps the ultimate sign that positive psychology has come of age: the "Live Happy" iPhone application, available free on iTunes.